AN ALTERED PATTERN OF RNA SYNTHESIS IN SERIALLY PROPAGATED HUMAN DIPLOID CELLS

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Abstract
Human diploid fibroblasts regularly developed profound and lasting changes in the pattern of RNA synthesis, which usually appeared after approximately 20 cell generations in culture. There was a marked decrease in the rate of C14-uridine incorporation into rapidly labeled nuclear 35-45S RNA and into ribosomal RNA, and the ribosomal RNA content decreased by approximately 50%. Coicidentally, a new species of rapidly labeled RNA appeared, sedimenting at 14-20S, with a guanine + cytosine content of 33%, located largely if not entirely in the cytoplasm, and tending to concentrate at the surface of the cell. In cytoplasmic extracts of late passage cells pulsed for 20 min., this RNA was localized in "heavy" cytoplasmic structures sedimenting at > 350S, from which it was released by treatment with deoxycholate and RNase, but not RNase alone. There was also a marked difference between the 2 cell types in the degree to which RNA synthesis was inhibited in confluent cul-tures. In early passage cells, C14-uridine incorporation into both RNase-sensitive and -resistant structures was almost abolished; only single ribosomes and subunits were continuingly labeled at ap-proximately 30% the rate in growing cultures; and the over-all rate of RNA synthesis was reduced by more than 90%. In late pas-sage cultures, however, uridine incorporation into the new 14-20S RNA and into RNase-resistant cytoplasmic structures was not markedly decreased by cell crowding, even though cell growth was curtailed to the same degree as in early passage cells.

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